Don't have a VW factory service manual at hand, but this practice seems to continue in the VW community, for example, this VW forum post ...
"_Re: How difficult is it to replace 16V injector o rings (2drJettaVR6) _
_11-06-2002 03:59 PM #12 _
Easy, use one of those pry bars used to pry wood and nails. Support the injector from the bottom with your hand and pry it out. to insert the new Orings, let them soak in fuel for an hour or so, , get the needle nosed pliers, slide the Oringon the pliers, open the pliers enough to have the injector between the arms and with a little fuel, slide the Oring over to the injector. "
And yes, it does cause them to swell a little. That's the idea, to make the seal firmly. And the gasoline on the surface provides some lube to get them to seat. I expect soaking in oil would work too, but might make them slippery and therefore easier to be pulled out by accident, or blown out were the engine to backfire into the intake manifold. That was more of a conern with the mechanical fuel injection system, because there was no fuel rail per se, each of the injectors were connected to the fuel distributor with individual hoses, so there was no force besides the o ring holding them in place. The fuel rail method has the fuel rail providing some mechanical support to hold them in place.